diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 9e8d6a674b..30d35e56c8 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -21,7 +21,7 @@ function File(opts) {
 File.declarations = ["extends", "class-props", "slice"];
 
 File.normaliseOptions = function (opts) {
-  opts = opts || {};
+  opts = _.cloneDeep(opts || {});
 
   _.defaults(opts, {
     whitespace: true,